Hi there. Thanks for your questions and for your close reading of the game. I hope I can answer all of your questions:
1) who decide what challenge the community will face? does it need to be solved during one scene? and if not, in what scene should be the dices be launched?
The narrator decides which challenge the community faces.
If the challenge is not resolved during "their" scene,
the next narrator(s) can build on it.
The dice are rolled whenever someone can do anything against a challenge. (No matter when the challenge was introduced. It persists until overcome)
2) "Regain lost Hope Points by spending a day connecting with others", does this implies that there is a day count? or does a scene count as a day?
This is meant as a narrative prompt.
Whenever you feel you've had a full day of connection, you can regain lost Hope Points. (Think a day at the park with a long picnic, going on a trip together, talking about your hopes and dreams until late at night, spending a day writing protest songs together...)
A scene is meant as a deliberately flexible unit of time.
Depending on the scenario you come up with, a scene can be an entire day in a months long battle against the landlord
or
a small increment of time during a tumultuous day of protest or other forms of political action.
3) In "advancment" it talks about "sessions". Does the game work well in one shots or is bettere in campaigns?
It depends on your group. Since the game rules are quite brief, you would have to come up with a lot of the frameworks for a longer campaign yourself.
I am however working on an expanded version of the game incl. modules for the communal space and a "bestiary" of challenges (landlords, bureaucrats, thugs, investors...)
You can sign up for the mailing list under the community section.
I hope I could answer all of your questions.
Fe